1. Field of the Invention
The present invention relates to improvement in a method for performing transient operation on a dryer steam pressure at a time of papermaking exchange in a papermaking machine.
2. Description of the Prior Art
FIG. 1 shows a configuration diagram of a typical papermaking machine. In this figure, pulp material is discharged from a stock inlet 61 to a wire part 62. The wire part 62 is moved in a direction of arrow A by rotating rolls 621. The pulp material discharged in the wire part 62 is filtrated so that a web (paper) is formed. The web thus formed is conveyed to a press part 63 where it is further squeezed.
The web which has been squeezed in the press part 63 is conveyed to a pre-dryer 64. Disposed in the pre-dryer 64 are many steam drums 641, which are heated by steam introduced therein. The web is conveyed so as to pass through the steam drums sequentially while it is being wound on the steam drums. In the course of this conveyance, the web is dried until a predetermined moisture percentage or moisture content in the web is achieved.
After the dried web is subjected to such a size treatment as size (coating agent) application in a size press 65, it is further dried in an after-dryer 66, it is taken up or rolled as a product such as denoted by reference numeral 67. Incidentally, the after-dryer 66 has substantially the same structure as that of the pre-dryer 64.
Reference numerals 68 and 69 denote BM meters which detect weightings, moisture contents or the like of the webs which have just been discharged from the pre-dryer 64 and from the after-dryer 66, respectively. The detected values are input into a control device (not shown). The control device controls a discharge amount of pulp material discharged into the wire part 62, or an amount of steam introduced into the steam drums in the pre-dryer 64 and the after-dryer 66, a papermaking speed and the like such that a product to be obtained meets specification values which have been determined in advance. Conventionally, a papermaking exchange control has been also employed for making different products in a continuous manner.
In the papermaking exchange control, since a product obtained during papermaking exchange where exchange is performed from a paper product making to another paper product making becomes broke out of a standard, such a papermaking exchange time should be reduced as much as possible in order to improve an operating efficiency. In order to solve the problem, there has been disclosed an invention about a method for estimating a setting value of steam pressure to be applied after a papermaking exchange according to simulation in Japanese Patent No. 3094798. The abstract of this invention will be explained below.
In the invention described in Japanese Patent No. 3094798 publication, using an iron mode where the steam drums in the pre-dryer 64 and the after-dryer 66 are arranged generally flat, contacting states among the steam drums, the web, and canvases wound on the steam drums in an endless manner are classified to five patterns to derive heat transfer differential equations of respective patterns, the differential equations are converted to difference equations, and a setting value of steam pressure is estimated by solving the difference equations.
Heat transfer differential equations of a pattern where the steam drum, the web and the canvas come into contact with one another in this order are represented in the following equations (1) to (3).                                                         L              D                        ·                          ρ              D                        ·                          C              D                                ⁢                                    ⅆ                                                T                  1                                ⁡                                  (                  t                  )                                                                    ⅆ              t                                      =                                            h              s                        ·                          (                                                                    T                    s                                    ⁡                                      (                    t                    )                                                  -                                                      T                    1                                    ⁡                                      (                    t                    )                                                              )                                -                                    h              DW                        ·                          (                                                                    T                    1                                    ⁡                                      (                    t                    )                                                  -                                                      T                    2                                    ⁡                                      (                    t                    )                                                              )                                                          (        1        )                                                                    L              W                        ·                          ρ              W                        ·                          C              W                                ⁢                                    ⅆ                                                T                  2                                ⁡                                  (                  t                  )                                                                    ⅆ              t                                      =                                            h              DW                        ·                          (                                                                    T                    1                                    ⁡                                      (                    t                    )                                                  -                                                      T                    2                                    ⁡                                      (                    t                    )                                                              )                                -                                    h              WC                        ·                          (                                                                    T                    2                                    ⁡                                      (                    t                    )                                                  -                                                      T                    3                                    ⁡                                      (                    t                    )                                                              )                                -                      Evapo            ⁡                          (                                                T                  2                                ,                                  T                  W                                            )                                                          (        2        )                                                                    L              C                        ·                          ρ              C                        ·                          C              C                                ⁢                                    ⅆ                                                T                  3                                ⁡                                  (                  t                  )                                                                    ⅆ              t                                      =                                            h              WC                        ·                          (                                                                    T                    2                                    ⁡                                      (                    t                    )                                                  -                                                      T                    3                                    ⁡                                      (                    t                    )                                                              )                                -                                    h              a                        ·                          (                                                                    T                    3                                    ⁡                                      (                    t                    )                                                  -                                                      T                    a                                    ⁡                                      (                    t                    )                                                              )                                                          (        3        )            
where respective parameters in the above equations (1) to (3) are as follows:
LD: Drum thickness (m)
Lw: web thickness (m)
Lc: canvas thickness (m)
Ts: in-drum steam temperature (xc2x0 C.)
T1: drum surface temperature (xc2x0 C.)
T2: web (paper) temperature (xc2x0 C.)
T3: canvas temperature (xc2x0 C.)
Ta: in-hood air dry-bulb temperature (xc2x0 C.)
CD: specific heat of drum (kJ/(kgxc2x7xc2x0 C.)
Cw: specific heat of web (kJ/(kgxc2x7xc2x0 C.)
Cc: specific heat of canvas (kJ/(kgxc2x7xc2x0 C.)
xcfx81D: density of drum (kg/m3)
xcfx81w: density of web (kg/m3)
xcfx81c: density of canvas (kg/m3)
hs: heat transfer coefficient between in-drum steam and drum surface (kJ/(m2xc2x7secxc2x7xc2x0 C.))
hDW: heat transfer coefficient between drum surface and web (kJ/(m2xc2x7secxc2x7xc2x0 C.))
hwc: heat transfer coefficient between web surface and canvas (kJ/(m2xc2x7secxc2x7xc2x0 C.))
ha: heat transfer coefficient between canvas and in-hood air (kJ/(m2xc2x7sec xc2x0 C.))
FIG. 2 is a table showing the respective parameters in a collecting manner.
In the above equation (2), Evapo (T2, Tw) is a function representing evaporation calorie taken away from a web by moisture evaporation, and it is represented as the following equation (4).
Evapo(T2,TW)=V(MPABS)xc2x7Kxc2x7(P(T2)xe2x88x92P(TW))xc2x7SB(T2)(kJ/(m2xc2x7sec))xe2x80x83xe2x80x83(4)
where P(T) is a saturated steam pressure (kPa) at a temperature T(xc2x0 C.); SB(T) is a heat of vaporization (kJ/H2 Okg) at a temperature T(xc2x0 C.); Tw is in-hood air wet-bulb temperature (xc2x0 C.); V(MPABS) is a function representing moisture evaporation intensity in an absolute moisture percentage MPABS (incidentally, 0.0xe2x89xa6V(MPABS)xe2x89xa61.0 (unit free); and K is a drying speed coefficient (H2 Okg/(m2xc2x7secxc2x7kPa)).
In the invention described in Japanese Patent No. 3094798, heat transfer differential equations about contacting patterns other than the above contacting pattern are given, but explanation thereof will be omitted for avoiding complexity. The differential equations (1) to (3) are rewritten to derive difference equations by differentiating time by a ticked time period xcex94t determined according to a papermaking speed, the circumference of a steam drum, and the like, so that numerical values are obtained from the difference equations. Since the web is moved from an upstream position to a downstream position according to time lapse, the temperature of the web on the steam drum can be calculated from the numerical values of the difference equations.
On the basis of the above equation (4), Evapo MP(T2, Tw)(H2 Okg/(m2xc2x7sec)) which is evaporated moisture content per unit area and unit time from the web can be represented by the following equation (5).
EvapoMP(T2,TW)=V(MPABS)xc2x7Kxc2x7(P(T2)xe2x88x92P(TW))(H2Okg/(m2xc2x7sec))xe2x80x83xe2x80x83(5)
Using this equation, the absolute moisture percentage MPABS(j) (j=1, . . . , N) of the web after elapse of a ticked time period xcex94t can be calculated according to the following equation (6).                                           MP            ABS                    ⁡                      (                          j              +              1                        )                          =                                            MP              ABS                        ⁡                          (              j              )                                -                                                                      10                  3                                ·                                  EvapoMP                  ⁡                                      (                                                                  T                        2                                            ,                                              T                        W                                                              )                                                  ·                Δ                            ⁢                              xe2x80x83                            ⁢              t                        BD                                              (        6        )            
where BD is an absolute dry weighting (g/m2); xcex94t is a ticked time period (sec); and MPABS(j)(j=1, . . . , N) is an absolute moisture percentage at a divided mesh position j.
On the basis of this absolute moisture percentage MPABS(j), a relative moisture percentage MP(j)(j=1, . . . , N)(%) can be calculated from the following equation (7).                               MP          ⁡                      (            j            )                          =                              100            ·                                                            MP                  ABS                                ⁡                                  (                  j                  )                                                            1                +                                                      MP                    ABS                                    ⁡                                      (                    j                    )                                                                                ⁢                      xe2x80x83                    ⁢                      (            %            )                                              (        7        )            
where MP(j)(j=1, . . . , N) is a relative moisture percentage (%) at a divided or split mesh position j.
FIG. 3 is a flowchart of an algorithm for performing simulation of a steady state using the above equations (1) to (7) to obtain a drying speed coefficient. In this figure, operation conditions, i.e., current papermaking speed (m/min), set value of weighting (g/m2) and set value of moisture percentage (%) are first taken in. Next, a ticked time period for difference calculation xcex94t is determined on the basis of the papermaking speed, the circumferential length of the steam drum, and then the steam temperature in the drum Ts(j)(j=1, . . . , N) is calculated from the set value of the steam pressure in the dryer using a saturated steam pressure curve. Incidentally, N is the number of division meshes.
Subsequently, using the above equations (1) to (7) and the difference equations derived therefrom, the drum temperature T1(j) (j=1, . . . , N), the web temperature T2(j)(j=1, . . . , N), the canvas temperature T3(j)(j=1, . . . , N), and the relative moisture percentage of web MP(j)(j=1, . . . , N) are calculated. Then, a determination is made about whether or not convergence occurs between the relative moisture percentage of web MP(N) at the final cylinder and the actually measured value MPMEASURE obtained in a moisture meter. That is, when an absolute value of a difference between MP(N) and MPMEASURE is smaller than a predetermined value EP, a determination is made that convergence has occurred.
xe2x80x9cEPxe2x80x9d is Estimated (moisture) Percentage.
When convergence does not occur, the drying speed coefficient K is corrected by xcex94K, and the drum temperature, the web temperature, the canvas temperature, and the relative moisture percentage of web are calculated again. Also, when the convergence occurs, the drying speed coefficient K, the values of the drum temperature T1(j), the web temperature T2(j), the canvas temperature T3(j) and the relative moisture percentage of web MP(j) are determined to the values obtained at this time to terminate the steady state simulation.
According to the steady state simulation mentioned above, the drying speed coefficient K is adjusted such that the absolute moisture percentage at the final cylinder approaches to the actually measured value. Next, a estimation of an optimal steam pressure setting value in an operational state after papermaking exchange is performed according to a steam pressure estimating simulation. This steam pressure estimating simulation will be explained with reference to a flowchart in FIG. 4.
In FIG. 4, operational conditions after papermaking exchange, namely, a papermaking speed (m/min), a set value of weighting (g/m2), and a set value of moisture percentage (%) are first taken in. Then, a ticked time period xcex94t which is applied for difference calculation is determined on the basis of the papermaking speed, the circumferential length of the drum and the like. Subsequently, the in-drum steam temperature Ts(j)(j=1, . . . , N) is calculated according to the dryer steam pressure set value P(kPa) using the saturated steam pressure curve. Here, N is the number of divided meshes.
Next, using the drying speed coefficient K in the final cylinder determined in the steady state simulation, numerical calculations are performed according to the above equations (1) to (7) and the difference equations to calculate the drum temperature T1(j)(j=1, . . . , N), the web temperature T2(j)(j=1, . . . , N), the canvas temperature T3(j)(j=1, . . . , N), and the relative moisture percentage of web MP(j)(j=1, . . . , N).
Then, the value of the relative moisture percentage of web MP(N) at the final cylinder and a moisture percentage set value MPTARGET after papermaking exchange are compared with each other to make a determination about convergence by a method similar to the case of the steady state simulation. When the convergence does not occur, the drum temperature, the web temperature, the canvas temperature, and the relative moisture percentage of web are calculated again while the set value of the dryer steam pressure P is corrected by a constant value xcex94P. When the convergence occurs, the steam pressure set value P obtained at this time is decided to terminate the steam pressure estimating simulation.
However, there is following drawbacks in such a method for estimating steam pressure after papermaking exchange in a papermaking machine.
It is considered that in-hood air is a fixed volume of air contained in a chamber, so-called dryer hood, isolated from outside air and the in-hood air dry-bulb temperature Ta varies according to the canvas temperature T3. For example, when the steam pressure set value is increased after papermaking exchange, the drum temperature T1, the web temperature T2, and the canvas temperature T3 are increased according to the above equations (1) to (3) so that the dry-bulb temperature Ta is also increased.
However, the invention described in Japanese Patent No. 3094798 is configured such that a fixed value which is not changed before and after papermaking exchange is employed as the dry-bulb temperature Ta, and the numerical value of the simulation is obtained using the fixed value as a boundary condition. Also, the invention has such a configuration that the same value is employed in both the steady state simulation and the steam pressure estimating simulation. For this reason, there is such a drawback that, when the steam pressure set value is increased after papermaking exchange, a steam pressure higher than a necessary steam pressure is estimated. Furthermore, there is such a drawback that, when the steam pressure set value is decreased after papermaking exchange, a steam pressure lower than an actual one is estimated.
Accordingly, an object of the present invention is to provide a method for estimating a dryer steam pressure in a papermaking machine, where for each simulation a dry-bulb temperature used in the simulation is calculated, and an apparatus therefor.